Abrasive articles are extensively used in numerous fabrication and dressing operations, such as abrasive machining and finishing. Depending on the intended use, abrasive articles can be formed either as an abrasive dispersed in a binder or resin matrix to form a bonded solid shape, or as an abrasive which is bonded to a suitable substrate such as metal or paper. Of interest here is the former category, which includes sharpening stones, honing stones, grinding wheels and the like. This form of abrasive article is particularly suited for rough and precision grinding.
Abrasive articles are produced by firing a greenware article having the shape of the desired final product. Generally, the greenware article is made by combining abrasive particles, a temporary binder and a permanent binder in a carrier medium which is capable of suitably suspending, dispersing and wetting the abrasive particles within the abrasive mixture. The permanent binder holds the abrasive particles together after firing when the abrasive article is in its finished form. The type of permanent binder used can be organic or inorganic, depending upon the properties desired.
The temporary binder is necessary to provide the greenware with sufficient strength for retaining its shape during handling and processing prior to its firing. Dextrin is commonly used as the temporary binder. However, poly(ethyloxazoline) has been shown to have advantageous characteristics over dextrin, as reported in U.S. Pat. No. 4,830,994 to Schuetz and U.S. Pat. No. 4,867,759 to Tiefenbach, Jr. et al, both of which are assigned to the assignee of the present invention. Both Schuetz and Tiefenbach, Jr. et al teach that the greenware article is formed at room temperature wherein the poly(ethyloxazoline) is a solid and acts to bind the abrasive grit.
After combining the ingredients, the abrasive mixture often undergoes processing, such as ball milling, to reduce the particle size of the abrasive particles and to further distribute the ingredients. The abrasive mixture is then dried by any conventional means, such as spray or pan drying, to form a substantially dry powder. The powder can then be formed into a greenware article using such well known methods as dry pressing, isostatic pressing, extrusion or slip casting. This step is of particular importance because the compression of the greenware article that occurs here substantially determines the density, porosity and strength of the final abrasive article produced.
Because density and strength are desirable characteristics in both the greenware and the abrasive article, additional ingredients, such as dispersants, plasticizers and lubricants, are often added to the abrasive mixture. Dispersants improve the solids to liquid ratio of the abrasive mixture and facilitate ball milling, plasticizers improve toughness and flexibility of the binder, while lubricants help to minimize particle-to-particle friction during pressing of the greenware article, thus promoting compressibility.
Following the forming step, the greenware article is then heated to remove or burn out the temporary binder. The required temperature for removing the temporary binder is dependent upon the type used. Details as to burn out time, temperature and pressure corresponding to a particular binder are readily ascertainable by those skilled in the art.
To further improve density and strength of the final abrasive article, the fired abrasive article may undergo further processing, such as hot pressing, hot isostatic pressing, sintering, hot isostatic forging, and the like. It is often necessary to machine, grind, cut or drill the final abrasive article to precisely achieve the desired form. This final step in processing is generally deferred until after firing because the greenware article is not sufficiently strong enough to withstand the rigors of such processing.
It can be readily appreciated that to reduce processing time and costs it is desirable to minimize the use of additional ingredients and post-firing pressing for purposes of producing a more dense abrasive article In addition, abrasive articles are difficult and expensive to machine, particularly after sintering, hot pressing, and hot isostatic pressing.
Accordingly, what is needed is a process for forming greenware articles which improves the compressibility of the abrasive mixture used in making the greenware article such that the density and strength of both the greenware article and the abrasive article are increased.